COMPUTATION OF DUAL-STREAM UNSTEADY JET FLOWS AND NOISE

The unsteady jet from an axisymmetric coaxial nozzle and the associated noise are computed by using a Lattice-Boltzmann Model (LBM) for high-speed subsonic flows. This method, recently developed by Exa, consists in coupling the standard LBM formulation for iso-thermal flows with a finite-difference solution of the entropy equation. With this new formulation it is possible to accurately simulate unsteady flows past complex geometries with significantly shorter turnaround times compared to conventional CFD methods based on the solution of the Navier-Stokes equations. In the present study, the primary and secondary jet exit Mach numbers are 0.87 and 0.90, respectively, the secondary jet Reynolds number is 2.8 million, and the primary to secondary temperature ratio is 2.7. The acoustic far-field is computed through a Ffowcs-Williams and Hawkings analogy applied to a fluid surface encompassing the plume. The accuracy of the numerical solution is evaluated against literature experimental data. The time-averaged velocity field and the root-mean-square velocity and pressure fields are shown to compare quite favourably with wind-tunnel measurements. The far-field noise spectra are also in fairly good agreement with the measurements, although affected by some lack of statistical convergence.